Power detectors are already well known in an extremely diverse range of embodiments. The most frequently used realisations contain either a thermal sensor, which generates an electrical measurement parameter proportional to the absorbed electrical power, or they contain a diode regulator in a one-way or two-way circuit, which supplies an electrical output parameter equivalent to the voltage at the terminal resistance, from which the power to be measured can then be determined.
In particular, the demands of the communications standards and the second and third generations of mobile telephones have led to the development of a new group of power detectors with a substantially greater dynamic range for modulated signals.
A power detector of this kind is known, for example, from the previously unpublished DE 100 39 665 of the applicant. In the case of the power detector described in this application, a stripline without electrical isolation is connected directly to the input terminal. The stripline terminates with a power distributor. A detector diode, which detects the positive and the negative half-wave of the measured signal in each case is disposed at the input and output ends of the stripline. The detector diodes can be connected to a difference amplifier in order to register the measured signal. As described in the application, a spatial isolation of the detector diodes has the advantage that the arrangement is less sensitive to reflections. The difference between the output voltages at the two detector diodes is therefore less dependent on the adaptation of the power distributor and the parasitic diode capacities. Other measurement branches with series connected attenuation stages are connected to the power distributor, so that a relatively large dynamic range is achieved in combination.
However, if the measured signal is associated with a direct-voltage component, a considerable measurement error occurs because of the direct-current-resistance of the stripline connecting the detector diodes and the associated drop in voltage.
Hitherto, it has been conventional to suppress the direct-voltage components of the measured signal with an isolating capacitor arranged between the high-frequency terminal and the detector diodes. An isolating capacitor of this kind is used, for example, in U.S. Pat. No. 4,943,764. Furthermore, with the solution proposed in this document, several diode pairs isolated from one another by power distributors are used to increase the dynamic range. However, the detector diodes pick up the two half-waves at the same measurement point. Accordingly, the relative insensitivity to reflections, which results from a spatial isolation of the detector diodes, as suggested in DE 100 39 665, is not available in this context.
The use of an isolating capacitor to suppress the direct-voltage component of the measured signal has several disadvantages. Firstly, the capacity of the isolating capacitor must be relatively large, in order to achieve a low lower-threshold frequency thereby allowing a broadband realisation of the power detector. This leads to a mechanically relatively large structure. Secondly, the isolating capacitor must be integrated in a low-reflection high-frequency line for microwave applications. This leads to additional interference reflections and therefore also to greater measurement errors.